This invention relates to memory elements and associated pass transistors, and more particularly, to memory element circuitry with stressed transistors.
Integrated circuits often contain volatile memory elements. Typical volatile memory elements are based on cross-coupled inverters (latches) and are used to store data. Each memory element can store a single bit of data. In integrated circuits such as programmable integrated circuits, volatile memory elements are used to store configuration data.
Memory elements are typically arranged in arrays. Data may be loaded into the memory elements of the array and may be read from the memory elements of the array using data lines. Once loaded, the output of a memory element in a programmable integrated circuit can serve as a static control signal that is applied to the gate of an associated pass transistor. Blocks of programmable logic can be customized by turning on and off appropriate pass transistors in this way.
To ensure satisfactory high-speed operation, it may be desirable to overdrive the pass transistors. In a typical overdrive scheme, pass transistors are controlled using larger voltage levels than are normally used in powering logic gates. To accommodate overdriven control signals, pass transistors on programmable integrated circuits can be fabricated using low-power (LP) devices. LP devices have thicker gate oxide than general purpose (G) devices and are therefore able to accommodate elevated gate voltages.
In addition to G devices and LP devices, memory circuitry on a programmable integrated circuit may have input-output (I/O) devices that are formed using gate oxides that are thicker than those of G devices and thicker than those of LP devices. This type of conventional arrangement involves a triple gate oxide process (i.e., separate masks are used for the G gate oxide, the LP gate oxide, and the I/O gate oxide). Compared to a dual gate oxide process (e.g., a process that involves two different gate oxide thicknesses), the triple gate oxide process requires extra masks and more complex processing.
It would therefore be desirable to be able to provide improved memory element circuitry for integrated circuits such as memory element circuitry for programmable integrated circuits that can be fabricated using dual gate oxide process.